1. Field of the Invention
The present invention relates generally to an oxide light emitting layer used for organic electroluminescent (EL) devices, and more particularly to a phosphor thin film used for a light emitting layer and an EL panel using the same.
2. Background Art
In recent years, thin-film EL devices have been increasingly studied for compact or large yet lightweight flat displays. A monochromatic thin-film EL display using a phosphor thin film comprising manganese-added zinc sulfide for yellowish orange light emission has already been practically used in the form of a double-insulating structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. Referring to FIG. 2, a lower electrode 5 is formed in a given pattern on a substrate 1, and a first insulating layer 2 is formed on the lower electrode 5. On the first insulating layer 2, there are provided a light emitting layer 3 and a second insulating layer 4 in this order. An upper electrode 6 is formed on the second insulating layer 4 in such a given pattern as to form a matrix circuit with the lower electrode 5.
To accommodate well to personal computer displays, TV displays and other displays, color displays are absolutely needed. Thin-film EL displays using a sulfide fluorescent material thin film are excellent in reliability and resistance to environmental conditions. At present, however, they are thought of as being unsuitable for color display purposes, because the properties of an EL fluorescent material for emitting the three primary colors or red, green and blue are less than satisfactory. Candidates for a blue emitting fluorescent substance are SrS:Ce where SrS is used as a matrix material and Ce as a luminescent center and ZnS:Tm, candidates for a red emitting fluorescent substance are ZnS:Sm and CaS:Eu, and candidates for a green emitting fluorescent substance are ZnS:Tb, CaS:Ce, etc, and studies thereof are now under way.
These fluorescent materials for emitting the three primary colors, viz., red, green and blue have problems in conjunction with light emission luminance, efficiency, color purity, etc., and so color EL panels are still on impractical levels. For blue in particular, relatively high luminance is obtained using SrS:Ce. However, such luminance is still unsatisfactory for blue applied to full-color displays, with chromaticity shifted to a green side. Thus, much improved blue emitting layers are in great demand.
To provide a solution to these problems, thiogallate or thioaluminate blue fluorescent substances such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu have been developed, as set forth in JP-A""s 07-122364 and 08-134440, Shingaku Giho EID98-113, pp. 19-24, and Jpn. J. Appl. Phys. Vol. 38, (1999), pp. L1291-1292. These thiogallate fluorescent substances offer no problem in connection with color purity, but have a low luminance problem. In particular, it is very difficult to obtain uniform thin films because such materials have a multiple composition. Poor crystallizability due to poor composition controllability, defects due to sulfur release, contamination with impurities, etc. appear to be leading factors for failures in obtaining thin films of high quality, and so resulting in no luminance increase. Thioaluminate in particular has great difficulty in composition controllability.
To achieve full-color EL panels, fluorescent materials capable of emitting blue, green and red light in a stable fashion and at low costs and their fabrication process are needed. However, phosphor thin films must be fabricated by separate processes depending on their type, because the chemical or physical properties of matrix materials for the phosphor thin films and luminescent center materials differ from material to material as mentioned above. For instance, with a film formation process capable of obtaining high luminance with one single material, it is impossible to increase the luminance of a phosphor thin film of other color. Given a full-color EL panel fabrication process, a plurality of different film formation systems are thus needed. As a result, the fabrication process increases in complexity, with an increasing panel fabrication cost.
The EL spectra of the aforesaid blue, green and red EL phosphor thin films are all broad. When they are used for a full-color EL panel, the RGB necessary for the panel must be cut out of the EL spectra of the EL phosphor thin films using separate filters. The use of such filters does not only make the fabrication process much more complicated, but also offer the gravest problem, viz., luminance drops. Extraction of RGB using filters causes practically unacceptable losses of 10 to 50% of the luminance of the blue, green and red EL phosphor thin films.
To provide a solution to the aforesaid problems, there is an increasing demand for red, green and blue fluorescent thin-film materials capable of emitting light at enhanced luminance yet with improved color purity as well as a fluorescent matrix material and a luminance center material which can ensure enhanced luminance using the same film formation method or system and are similar to each other in terms of chemical or physical properties.
One object of the present invention is to provide a phosphor thin film which can dispense with any filter and has satisfactory color purity, and is particularly well fit for RGB full-color ELs and its fabrication process as well as an EL panel.
Another object of the present invention is to simplify a full-color EL panel production process, thereby providing a phosphor thin film which is less susceptible to luminance variations and can be produced in improved yields and so at lower costs and its fabrication process as well as an EL panel.
Such objects are achievable by the following embodiments (1) to (11) of the invention.
(1) A phosphor thin film comprising a matrix material containing as a main component an alkali earth aluminate that is an oxide and containing sulfur, and further containing an element that provides a luminescent center.
(2) The phosphor thin film according to (1) above, which is represented by
AxAlyOzSw:Re
where Re is a rare earth element, A is at least one element selected from Mg, Ca, Sr and Ba, x=1 to 5, y=1 to 15, z=3 to 30, and w=3 to 30.
(3) The phosphor thin film according to (1) above, wherein the molar ratio, S/(S+O), of the sulfur element contained therein with respect to an oxygen atom in said matrix material is in the range of 0.01 to 0.5.
(4) The phosphor thin film according to (2) above, wherein 1.5xe2x89xa6y/xxe2x89xa63.0.
(5) The phosphor thin film according to (4) above, wherein S/(S+O)=0.7 to 0.9.
(6) A phosphor thin film represented by
AxAlyOzSw:Re
where Re is a rare earth element, A is at least one element selected from Mg, Ca, Sr and Ba, x=1 to 5, y=1 to 15, z=3 to 30, and w=3 to 30 provided that 5xe2x89xa6y/xxe2x89xa67.
(7) The phosphor thin film according to (1) above, wherein said luminescent center Re is any one of Eu, Tb and Sm.
(8) An electroluminescent panel comprising a phosphor thin film as recited in (1) above.
(9) A phosphor thin film fabrication process comprising steps of:
forming a sulfide thin film containing sulfur and a luminescent center for a matrix material precursor, and
annealing the sulfide thin film in an oxidizing atmosphere to introduce oxygen therein, thereby obtaining a phosphor thin film as recited in (1) above.
(10) A process for fabricating a phosphor thin film as recited in (1) above by an evaporation process, which comprises steps of:
introducing an oxygen gas in a vacuum chamber in which, at least, an aluminum sulfide evaporation source and an alkali earth sulfide evaporation source with a luminescent center added thereto are disposed, and
evaporating aluminum sulfide and an alkali earth sulfide material from the respective evaporation sources to combine the respective feed materials with the oxygen gas during the deposition thereof on a substrate, thereby obtaining said phosphor thin film.
(11) A process for fabricating a phosphor thin film as recited in (1) above by an evaporation process, which comprises steps of:
introducing a hydrogen sulfide gas in a vacuum chamber in which, at least, an aluminum sulfide evaporation source and an alkali earth sulfide evaporation source with a luminescent center added thereto are disposed,
evaporating aluminum sulfide and an alkali earth sulfide material from the respective evaporation sources,
combining the respective feed materials with the hydrogen sulfide gas during the deposition thereof on a substrate, thereby obtaining a sulfide phosphor thin film, and
annealing the sulfide phosphor thin film in an oxidizing atmosphere.